A high-purity synthetic quartz glass used for manufacturing optical fiber base materials and the like can be obtained in the following manner. A glass material such as silicon tetrachloride is subjected to flame hydrolysis in an oxyhydrogen flame, so that glass particles (silica powders) are produced. The produced glass particles are deposited onto a starting member such as a quartz glass while the starting member is rotated, so that a porous glass base material is manufactured. The porous glass base material is sintered and vitrified into a transparent glass.
There are several different methods to manufacture a porous glass base material, such as the vapor phase axial deposition (VAD) method, the outside VAD method, and the outside vapor deposition (OVD) method. FIG. 1A shows the VAD method. To be specific, a glass material is supplied to a burner 2 so that glass particles are produced. The produced glass particles are deposited onto the end of a starting member 1. Here, a porous glass base material 4 is grown in the axial direction of the starting member 1 in such a manner that a shaft 3 coupled to the starting member 1 is moved upwards while rotated via a hanging mechanism (not shown). FIG. 1B shows the outside VAD method, whereby the porous glass base material 4 is grown in the axial direction of the starting member 1 along the lateral surface of the starting member 1. FIG. 1C shows the OVD method, whereby the porous glass base material 4 is grown in the radial direction of the starting member 1 in such a manner that the burner 2 is reciprocated in a relative manner along the lateral surface of the starting member 1. Here, it should be noted that the number of burners 2 is one in FIGS. 1A to 1C, but can be appropriately increased as needed.
Here, some of the produced glass particles are not deposited onto the starting member to manufacture the porous glass base material. Part of the non-deposited glass particles keep floating within the process chamber, to be attached and deposited onto the wall of the process chamber as soot, and the rest are exhausted outside the process chamber together with the exhaust gas.
Here, a piece of the soot may come off the wall and then be attached to the deposition surface of the porous glass base material which is being manufactured by deposition. The piece of the soot is turned into a bubble as a result of vitrification of the porous glass base material into a transparent glass. Such a bubble is present in a finished product, resulting in quality degradation.
To solve this problem in relation to the soot, the following technique is disclosed by Patent Document 1. A slit-like gas inlet 5 is provided, in the vicinity of the ceiling, in a lateral wall of a process chamber at the side where a burner 2a for a core, a burner 2b for heating the core, and burners 2c for a clad are provided, as shown in FIG. 2. In addition, an exhaust outlet 6 is provided in a lateral wall that opposes the gas inlet 5. This configuration produces an air curtain effect whereby a gas supplied from the gas inlet 5 flows into the exhaust outlet 6 along the ceiling. Accordingly, the soot to be formed on the ceiling can be significantly reduced.
In recent years, however, the size of optical fiber base materials has been on the rise because of demand for rationalized manufacture of optical fibers, cost reduction, and the like. This tendency makes it necessary to manufacture porous glass base materials of a large size, resulting in an increase in the amount of the glass material supplied. As the amount of the glass material supplied to the process chamber is increased, the amount of floating glass particles that are not deposited onto the starting member accordingly increases. Considering this, the technique disclosed by Patent Document 1 can not fully prevent the soot from being formed on the walls of the process chamber. Therefore, a piece of the soot may come off the walls and fall, to be attached to the deposition surface of the porous glass base material.
This drawback is clearly shown in FIG. 3, which illustrates the gas flow at the level of the gas inlet of FIG. 2. To be specific, since the gas flowing from the gas inlet is partly blocked by the porous glass base material, a swirling gas flow in the reverse direction is formed on a section of the ceiling which is closer to the exhaust outlet. This swirling gas flow keeps therein floating glass particles for a long time period, and the floating glass particles are attached as soot onto the ceiling.
Furthermore, the technique disclosed in Patent Document 1 can not reduce the amount of the soot formed on the lateral walls of the process chamber.
[Patent Document 1] Unexamined Japanese Patent Application Publication No. 2002-193633